The present invention relates to a twin master cylinder for a road vehicle hydraulic dual-circuit brake system and, more particularly, to such a system having a front-axle/rear-axle brake-circuit allocation, with master cylinders which are assigned respectively to a front-axle brake circuit and to the rear-axle brake circuit and which are arranged at a lateral distance next to one another in a common housing, with their central longitudinal axes extending in parallel. Each master cylinder is actuable via an arm of a pivotable rocker acting as a two-armed lever and with a variable ratio defined by the lengths of the rocker arms.
A twin master cylinder is described in German Offenlegungsschrift 1,655,266. In the conventional twin master cylinder, an actuating force acting on a rocker with a pedal step-up ratio is transmitted via a pedal tappet which is supported with its hemispherical end on a complementary hemispherical bottom of a conical sleeve. The sleeve widens on the pedal side and is inserted into a conical rocker recess of a form complementary to its outer contour. The actuating force exerted on the rocker in the axial direction is transmitted to the master-cylinder pistons via slender tappets which are supported with their hemispherical ends on the rocker side in conical recesses of the rocker and with their ends on the piston side in conical recesses of the master-cylinder pistons.
To make it possible to vary the ratio L1/L2 of effective lengths, L1 and L2, of the rocker arms which is determined by the radial distances of the supporting points of the tappets engaging on the master-cylinder piston from the supporting point of the pedal tappet on the rocker, the conical sleeve located on the pedal side has a symmetry differing from the axial symmetry and its arranged rotatably in the conical rocker recess of a form complementary to its outer contour. The master cylinder can thereby be set to a restricted extent to different values of the ratio of the brake forces which can be exerted via the two brake circuits. A setting made in this respect, for example to take into account a constructively predetermined axle-load ratio, is not subsequently changeable.
A setting of the ratio L1/L2 of the rocker-arm lengths taking into account different load states of the vehicle and different axle-load distributions resulting from these different load states so that relatively high rear-axle brake-force fractions can be utilized when the vehicle is fully loaded is not possible, at least not in a simple way.